1. Field of the Invention
The present invention relates generally to a mobile communication systems, and in particular, to a handoff method in a mobile communication system supporting transmission diversity for a base station.
2. Description of the Related Art
In a mobile communication system, a xe2x80x9ctransmission diversityxe2x80x9d function is generally used to multiplex a path for signals transmitted from a base station to a mobile station. This provides an increase in reliability of the received signals having the same transmission power at a receiving mobile station as at the base station. Here, the xe2x80x9cpathxe2x80x9d refers to a physical path through which a signal can be transmitted from a transmitter to a receiver. The physical path depends on several factors, such as propagation direction of a transmission antenna, polarization of a transmission signal, location of the transmission antenna, different carrier used on a frequency axis, and different transmit time on a time axis. The path does not necessarily mean a spatial path. A term xe2x80x9cmicro transmission diversityxe2x80x9d as used herein refers to a transmission diversity function implemented in one base station, and a term xe2x80x9cmacro transmission diversityxe2x80x9d refers to a transmission diversity function implemented within multiple base stations. A common transmission diversity function is the time switched transmission diversity (TSTD). The TSTD concept is explained in Korean Patent Application Serial No. 1998-5526 having a common assignee.
For simplicity, a base station having the transmission diversity function will be referred to herein as a TSTD base station, and a base station not supporting the transmission diversity function as a non-TSTD base station. Furthermore, a mobile station supporting the transmission diversity function will be referred to herein as a TSTD mobile station and a mobile station not supporting the transmission diversity function as a non-TSTD mobile station.
By way of example, reference will now be made to a prior art soft handoff operation in conjunction with FIGS. 5A and 5B in the case were a soft handoff operation occurs between the non-TSTD mobile station and two non-TSTD base stations.
Referring to FIG. 5A, a signal exchange occurs between a non-TSTD mobile station and two non-TSTD base stations A and B; that is, when the non-TSTD mobile station travels from a non-TSTD base station A (BS#A) to an adjacent non-TSTD base station B (BS#B). Immediately before the soft handoff operation, only the base station A (BS#A) transmits a signal and the mobile station receives the transmitted signal (320). During the soft handoff operation, the base stations A and B simultaneously transmit the same information and the mobile station assigns the fingers to the transmission paths from the respective base stations to receive the transmitted information (330). After completion of the soft handoff operation, only the base station B (BS#B) transmits a signal and the mobile station receives the transmitted signal (340).
FIG. 5B illustrates a timing chart of control signals and traffic signals between the base stations and the mobile station during the handoff operation shown by FIG. 5A. In step 511, the non-TSTD base station A and the non-TSTD mobile station communicate with each other in the non-TSTD mode of operation. During the communication, the mobile station examines in step 513 whether a receiving strength of a signal from the base station A is lower than a threshold to determine whether handoff is required. When the handoff is not required, the mobile station returns to step 511 and continuously communicates with the base station A.
Otherwise, when handoff is required, the mobile station proceeds to step 515 to send a handoff request to the base station A. The base station A then sends a handoff confirm message (or handoff approve message) to the mobile station in step 517. Here, the mobile station can make the handoff request directly by sending a control signal, or can make the handoff request indirectly. For example, for the direct handoff request the mobile station measures strengths of signals from the base stations adjacent thereto and sends an ID (identification) of a base station having the highest signal strength (herein, the base station A) to the base station A. However, for the indirect handoff request, the mobile station sends to the base station A not only a measured strength of a signal from the base station A but also the measured strengths of signals from the adjacent base stations having a received signal strength higher than a threshold, so as to enable the base station A to determine whether to perform the handoff to a handoff target base station (herein, the base station B) when the handoff is required.
In answer to the handoff request, the base station A inquires a base station controller (BSC) as to whether the base station B can accept the handoff request, and sends the inquiry result to the mobile station. At the same moment, the base station B can also send the mobile station information as to whether the handoff is acceptable, in step 519. When the handoff is approved, the base station A sends a signal in the non-TSTD mode of operation in step 521, and at the same time, the base station B also sends the same signal in the non-TSTD mode of operation in step 523. The mobile station receives the transmitted signals by assigning the fingers for the signals from the respective base stations, multiplies the received signals by the weights corresponding to qualities of the signals from the respective base stations using the combiner and accumulates the multiplied values, to thereby perform the soft handoff operation.
In the meantime, when the quality of a signal from the base station A decreases below a threshold, the mobile station sends a release request message for a channel connected to the base station A in step 525, and the base station A then sends an acknowledge message in answer to the request message and releases the channel in step 527. At the same time, the mobile station also discontinues receiving a signal from the base station A. Thereafter, in step 529, the mobile station communicates with the base station B in the non-TSTD mode of operation.
As described above, the soft handoff operation occurs between base stations and a mobile station without taking the TSTD function into consideration, thereby simplifying the handoff procedure. But, applying the TSTD function to the base station is advantageous in that a higher communication quality can be obtained with the same mean transmission power. However, the TSTD function increases the cost of the mobile communication system, since additional hardware is required. Therefore, in an effort to minimize cost, TSTD base stations are installed in heavy traffic areas and non-TSTD base stations are installed in light traffic areas. Accordingly, when the TSTD function is introduced to a non-TSTD base station installed in a light traffic area in an effort to increase the communication quality, a new handoff procedure is required; that is, the mobile station must be handed-off to a base station which supports the TSTD function. The present invention provides novel methods for performing handoff operations in this and other circumstances while minimizing cost and operational complexity.
It is, therefore, an object of the present invention to provide a soft handoff method which can be performed between a TSTD mobile station and two TSTD base stations.
It is another object of the present invention to provide a soft handoff method which can be performed when a TSTD mobile station travels from a non-TSTD base station to a TSTD base station.
It is further another object of the present invention to provide a soft handoff method which can be performed between a TSTD mobile station and two non-TSTD base stations.
It is still another object of the present invention to provide a soft handoff method which can be performed when a TSTD mobile station travels from a TSTD base station to a non-TSTD base station.
It is further still another object of the present invention to provide a soft handoff method which can be performed between a non-TSTD mobile station and two TSTD base stations.
It is further still another object of the present invention to provide a soft handoff method which can be performed when a non-TSTD mobile station travels from a non-TSTD base station to a TSTD base station.
It is further still another object of the present invention to provide a soft handoff method which can be performed between a non-TSTD mobile station and two non-TSTD base stations.
It is further still another object of the present invention to provide a soft handoff method which can be performed when a non-TSTD mobile station travels from a TSTD base station to a non-TSTD base station.
To achieve these and other objects, handoff methods are provided for a mobile communication system supporting transmission diversity. In one embodiment, a method is disclosed for performing a handoff operation when a non-TSTD mobile station travels from a non-TSTD base station A to an adjacent non-TSTD base station B. Immediately before the soft handoff, the base station A transmits a signal in the non-TSTD mode of operation, and the mobile station receives the transmitted signal. During the soft handoff operation, the base stations A and B alternately transmit the same data via selected antennas thereof in a macro-TSTD mode of operation, and the mobile station assigns fingers to the transmission paths from the respective base stations to receive the transmitted data. In this embodiment, two fingers are assigned all together. After completion of the soft handoff operation, the base station B operates in the non-TSTD mode and the mobile station also operates in the non-TSTD mode to receive data from the base station B.
During the handoff operation, the non-TSTD base station A and the non-TSTD mobile station communicate with each other in the non-TSTD mode of operation. During the communication, the mobile station examines whether a receiving strength of a signal from the base station A is lower than a threshold to determine whether handoff is required. When handoff is not required, the mobile station continuously communicates with the base station A. Otherwise, when handoff is required, the mobile station sends a handoff request to the base station A. The base station A then sends a handoff confirm message to the mobile station in response to the handoff request. Together with the handoff request, the mobile station sends information about the handoff target base station B and information about a transmission antenna, selected for the non-TSTD mode, out of the transmission antennas of the base station A. Upon receipt of the handoff request, the base station A inquires the base station controller (BSC) as to whether the base station B can accept the handoff, and sends the inquiry result to the mobile station. At the same moment, the base station B can also send the mobile station information as to whether the handoff is acceptable.
Thereafter, upon entering the soft handoff state, the base station A transmits a signal in the macro-TSTD mode of operation and at the same time, the base station B also transmits the same signal in the macro-TSTD mode of operation. The mobile station then receives the transmitted signals by assigning the fingers for the signals from the respective base stations, multiplies the received signals by the weights corresponding to qualities of the signals from the respective base stations using a combiner and accumulates the multiplied values, to thereby perform the soft handoff operation.
In the meantime, if the quality of a signal from the base station A decreases below a threshold, the mobile station sends a release request message for a channel connected to the base station A. The base station A then sends an acknowledge message in response to the request message and releases the channel. At the instant of sending the channel release request, the mobile station sends a macro-TSTD mode release request to the base station B, and the base station B sends an acknowledge message in response to the request. Upon receipt of the acknowledge message, the mobile station switches the receiving mode from the macro-TSTD mode to the non-TSTD mode and communicates with the base station B in the non-TSTD mode of operation.